127 research outputs found
A New Multiple Stellar System in the Solar Neighborhood
Adaptive optics corrected images obtained with the CIAO instrument at the
Subaru 8.2-meter telescope show the presence of two subarsecond companions to
the nearby (d=19.3 pc) young star GJ 900, which was previously classified as a
single member of the IC 2391 supercluster. The two companions share the same
proper motion as the primary and are redder. Their projected separations from
the primary are 10 AU and 14.5 AU for B and C, respectively. The estimated
masses for the two new companions depend strongly on the age of the system. For
the range of ages found in the literature for IC 2391 supercluster members
(from 35 Myr to 200 Myr), the expected masses range from 0.2 M to 0.4
M for the B component, and from 0.09 M to 0.22 M for
the C component. The determination of the dynamical mass of the faintest
component of GJ 900 will yield the age of the system using theoretical
evolutionary tracks. The apparent separations of the GJ 900 system components
meet the observational criterion for an unstable Trapezium-type system, but
this could be a projection effect. Further observations are needed to establish
the nature of this interesting low-mass multiple system.Comment: Scheduled for publication in the Astronomical Journal (August 2003
Collective relaxation of stellar systems revisited
The chaos in stellar systems is studied using the theory of dynamical systems
and the Van Kampen stochastic differential equation approach. The exponential
instability (chaos) of spherical N-body gravitating systems, already known
previously, is confirmed. The characteristic timescale of that instability is
estimated confirming the collective relaxation time obtained by means of the
Maupertuis principle.Comment: A & A (in press), 3 pages, to match the published versio
Hubble Space Telescope NICMOS Imaging of W3 IRS 5: A Trapezium in the Making?
We present Hubble Space Telescope NICMOS imaging of W3 IRS 5, a binary
high-mass protostar. In addition to the two protostars, NICMOS images taken in
the F222M and F160W filters show three new 2.22 micron sources with very red
colors; these sources fall within a region 5600 AU in diameter, and are
coincident with a 100 solar mass dense molecular clump. Two additional point
sources are found within 0.4'' (800 AU) of one of the high-mass protostars;
these may be stellar companions or unresolved emission knots from an outflow.
We propose that these sources constitute a nascent Trapezium system in the
center of the W3 IRS 5 cluster containing as many as five proto OB stars. This
would be the first identification of a Trapezium still deeply embedded in its
natal gas.Comment: accepted to ApJ letter
Young Globular Clusters and Dwarf Spheroidals
Most of the globular clusters in the main body of the Galactic halo were
formed almost simultaneously. However, globular cluster formation in dwarf
spheroidal galaxies appears to have extended over a significant fraction of a
Hubble time. This suggests that the factors which suppressed late-time
formation of globulars in the main body of the Galactic halo were not operative
in dwarf spheroidal galaxies. Possibly the presence of significant numbers of
``young'' globulars at R_{GC} > 15 kpc can be accounted for by the assumption
that many of these objects were formed in Sagittarius-like (but not
Fornax-like) dwarf spheroidal galaxies, that were subsequently destroyed by
Galactic tidal forces. It would be of interest to search for low-luminosity
remnants of parental dwarf spheroidals around the ``young'' globulars Eridanus,
Palomar 1, 3, 14, and Terzan 7. Furthermore multi-color photometry could be
used to search for the remnants of the super-associations, within which outer
halo globular clusters originally formed. Such envelopes are expected to have
been tidally stripped from globulars in the inner halo.Comment: 18 pages, with 2 figures, in LaTeX format; to appear in the
Astrophysical Journal in February 200
Development of a crank mechanism with a rolling element
В статье обосновывается замена кинематической пары поршень-шатун (низшая пара) на высшую в кинематической цепи кривошипно-шатунного механизма, что уменьшит количество избыточных связей, а это в свою очередь снизит трудоемкость изготовления звенев и повысит надежность механизма.This article explains replacement of a kinematic pair piston‐rod (lower pair) by the highest one, which reduces the number of redundant links. Thus will be reduced the complexity of manufacturing and increasing mechanism reliability
Geometrical Ambiguity of Pair Statistics. I. Point Configurations
Point configurations have been widely used as model systems in condensed
matter physics, materials science and biology. Statistical descriptors such as
the -body distribution function is usually employed to characterize
the point configurations, among which the most extensively used is the pair
distribution function . An intriguing inverse problem of practical
importance that has been receiving considerable attention is the degree to
which a point configuration can be reconstructed from the pair distribution
function of a target configuration. Although it is known that the pair-distance
information contained in is in general insufficient to uniquely determine
a point configuration, this concept does not seem to be widely appreciated and
general claims of uniqueness of the reconstructions using pair information have
been made based on numerical studies. In this paper, we introduce the idea of
the distance space, called the space. The pair distances of a
specific point configuration are then represented by a single point in the
space. We derive the conditions on the pair distances that can be
associated with a point configuration, which are equivalent to the
realizability conditions of the pair distribution function . Moreover, we
derive the conditions on the pair distances that can be assembled into distinct
configurations. These conditions define a bounded region in the
space. By explicitly constructing a variety of degenerate point configurations
using the space, we show that pair information is indeed
insufficient to uniquely determine the configuration in general. We also
discuss several important problems in statistical physics based on the
space.Comment: 28 pages, 8 figure
Diameters of Open Star Clusters
The present paper presents a tabulation of data on all 600 Galactic open
clusters for which it is presently possible to calculate linear diameters. As
expected, the youngest `clusters' with ages < 15 Myr, contain a significant
(greater than or equal to 20%) admixture of associations. Among
intermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, the median
cluster diameter is found to increase with age. Small compact clusters are rare
among objects with ages > 1.5 Gyr. Open clusters with ages > 1 Gyr appear to
form what might be termed a `cluster thick disk', part of which consistst of
objects that were probably captured gravitationally by the main body of the
Galaxy.Comment: Astronomical Journal, in pres
Investigating the Nature of Variable Class I and Flat Spectrum Protostars Using 2-4m Spectroscopy
In this study I present new K and L-band infrared photometry and 2-4m
spectra of ten Class I and flat spectrum stars forming within the Taurus dark
cloud complex. Nine sources have H {\it v}=0-1 S(1) emission, and some show
multiple H emission features in their K-band spectra. Photospheric
absorptions characteristic to low mass stars are detected in five of the
targets, and these stars were fit with models to determine spectral type,
infrared accretion excess veiling (r and r) and dust temperatures,
estimates of visual extinction and characteristics of the 3m water-ice
absorption. On average, the models found high extinction values, infrared
accretion excess emission with blackbody temperatures in the 900-1050K range,
and 3m absorption profiles best fit by water frozen onto cold grains
rather than thermally processed ice. Five techniques were used to estimate the
extinction toward the stellar photospheres; most gave vastly different results.
Analysis of emission line ratios suggests that the effect of infrared scattered
light toward some protostars should not be neglected. For stars that exhibited
Br in emission, accretion luminosities were estimated using relations
between L and Br luminosity. The young stars in this sample
were preferentially chosen as variables, but they do not have the accretion
dominated luminosities necessary to put them in their main stage of
mass-building. The characteristics of the 2-4m spectra are placed in the
context of existing multi-wavelength data, and five of the stars are more
consistent with reddened Class IIs or stars in transition between Class I and
II, rather than protostars embedded within massive remnant envelopes.Comment: Full resolution version available at:
http://www.ess.sunysb.edu/tracy/tbeck_mar07_AJ.pdf. Accepted for Publication
in the Astronomical Journal (March 2007
Deficit of wide binaries in the eta Chamaeleontis young cluster
We have carried out a sensitive high-resolution imaging survey of stars in
the young (6-8 Myr), nearby (97 pc) compact cluster around eta Chamaeleontis to
search for stellar and sub-stellar companions. Given its youth and proximity,
any sub-stellar companions are expected to be luminous, especially in the near
infrared, and thus easier to detect next to their parent stars. Here, we
present VLT/NACO adaptive optics imaging with companion detection limits for 17
eta Cha cluster members, and follow-up VLT/ISAAC near-infrared spectroscopy for
companion candidates. The widest binary detected is ~0.2", corresponding to the
projected separation 20 AU, despite our survey being sensitive down to
sub-stellar companions outside 0.3", and planetary mass objects outside 0.5".
This implies that the stellar companion probability outside 0.3" and the brown
dwarf companion probability outside 0.5" are less than 0.16 with 95%
confidence. We compare the wide binary frequency of eta Cha to that of the
similarly aged TW Hydrae association, and estimate the statistical likelihood
that the wide binary probability is equal in both groups to be < 2e-4. Even
though the eta Cha cluster is relatively dense, stellar encounters in its
present configuration cannot account for the relative deficit of wide binaries.
We thus conclude that the difference in wide binary probability in these two
groups provides strong evidence for multiplicity properties being dependent on
environment. In two appendices we derive the projected separation probability
distribution for binaries, used to constrain physical separations from observed
projected separations, and summarize statistical tools useful for multiplicity
studies.Comment: Accepted by ApJ. 13 pages, 10 figure
The metastasis associated protein S100A4: role in tumour progression and metastasis
The metastasis associated protein S100A4 is a small calcium binding protein that is associated with metastatic tumors and appears to be a molecular marker for clinical prognosis. Below we discuss its biochemical properties and possible cellular functions in metastasis including cell motility, invasion, apoptosis, angiogenesis and differentiation
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